Anisotropic polymer layer

ABSTRACT

The invention relates to an optical retardation film comprising two layers of an anisotropic polymer adjacent to each other or adjacent to both sides of a substrate, characterized in that each layer exhibits a tilted structure with an optical symmetry axis having a tilt angle θ relative to the plane of the layer, to a means to produce substantially linear polarized light comprising such an optical retardation film and to a liquid crystal display comprising such an optical retardation film.

This application is a divisional application of U.S. application Ser.No. 09/254,185, filed 2 Mar. 1999, now U.S. Pat. No. 6,291,035 B1,issued 18 Sep. 2001, which was the National Stage of InternationalApplication No. PCT/EP97/04827, filed 5 Sep. 1997.

SUMMARY OF THE INVENTION

The invention relates to an optical retardation film comprising twolayers of an anisotropic polymer that are adjacent to each other oradjacent to both sides of a common substrate, characterized in that eachlayer exhibits a tilted structure with an optical symmetry axis having atilt angle θ relative to the plane of the layer.

The invention further relates to a method of preparing such an opticalretardation film, to a means to produce substantially linear polarizedlight comprising such an optical retardation film and to a liquidcrystal display comprising a display cell and such an opticalretardation film.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 a to 1 c illustrate the structure of optical retardation filmsaccording to preferred embodiments of the present invention.

FIGS. 2 a and 2 b show a display device according to a preferredembodiment of the present invention.

FIGS. 3 a and 3 b show the retardation versus viewing angle of aninventive optical retardation film compared to optical retardation filmsof the state of the art, measured in two directions of observation.

FIG. 4 shows the normalized retardation versus wavelength for aninventive optical retardation film compared to optical retardation filmsof the state of the art.

FIG. 5 shows the retardation versus viewing angle of an inventiveoptical retardation film, measured in two directions of observation.

FIGS. 6 a and 6 b show the luminance versus viewing angle for aninventive combination of a broad waveband circular reflective polarizerand an inventive optical retardation film, compared to a combination ofa broad waveband circular reflective polarizer and different opticalretardation films of the state of the art.

The European Patent Application EP 0 606 940-A1 discloses a cholestericreflective polarizer that produces circular polarized light of a highluminance over a broad range of wavelengths. This polarizer can also becombined with a quarter wave foil or plate (QWF), which transforms thecircular polarized light transmitted by the cholesteric polarizer intolinear polarized light.

However, when a liquid crystal display comprising a cholestericpolarizer like that described in EP 0 606 940 is watched under anincreasing viewing angle, its optical properties, like e.g. theluminance and the contrast ratio, are often deteriorating.

It has therefore been desired to have available an optical retardationfilm that, when used together with a broad waveband cholesteric circularreflective polarizer or a combination comprising a circular reflectivepolarizer and a quarter wave foil, like e.g. described in EP 0 606 940,improves the optical properties of the above mentioned circularreflective polarizer or combination over a wide range of viewing angles.

One of the aims of the present invention is to provide an opticalretardation film having the properties described above. Another aim ofthe invention is to provide a means to produce substantially linearpolarized light comprising such an optical retardation film and abroadband circular reflective polarizer. Other aims of the presentinvention are immediately evident to the person skilled in the art fromthe following detailed description.

It has now been found that these aims can be achieved by providing anoptical retardation film comprising two layers of an anisotropic polymerthat are adjacent to each other or adjacent to both sides of asubstrate, characterized in that each layer exhibits a tilted structurewith an optical symmetry axis having a tilt angle θ relative to theplane of the layer.

Furthermore it has been found that an optical retardation film asdescribed above can also be used as a compensation film in conventionalLC displays, like for example TN, STN or active matrix driven (AMD) TNdisplays, in order to compensate the viewing angle dependence of theelectrooptical properties, like e.g. the contrast and grey scale, inthese displays.

A retardation film with a tilted molecular structure is described in theWO 96/19770-A1, whereas the WO 96/19770-A1 discloses a retardation filmwith a tilted structure wherein the tilt angle varies continuously in adirection normal to the film. However, there is no hint to the combineduse of such a retardation film with a broadband circular reflectivepolarizer in these documents.

One of the objects of the present invention is an optical retardationfilm comprising two layers of an anisotropic polymer that are adjacentto each other or adjacent to both sides of a common substrate,characterized in that each layer exhibits a tilted structure with anoptical symmetry axis having a tilt angle θ relative to the plane of thelayer.

In a preferred embodiment of the present invention the opticalretardation film exhibits a tilted structure wherein the tilt angles ofthe two layers vary from one another.

In another preferred embodiment the optical retardation film exhibits atilted structure wherein the tilt angle in each layer variescontinuously, i.e. assumes a splay configuration, wherein the preferredembodiment is characterized in that either the direction of thevariation or the amount of the starting values of the variation of thetilt angle are different between the two layers.

In the first layer according to this preferred embodiment the tilt angleθ varies, preferably continuously, in a direction normal to the layerfrom a minimum value θ_(min) on the side of the layer facing the secondlayer or, if present, the common intermediate substrate, to a maximumvalue θ_(max) on the opposite side of the layer, or the other way round.

In the second layer, the tilt angle varies, preferably continuously, ina direction normal to the layer, starting from a minimum value θ_(min)at the side of the layer facing the first layer or, if present, thecommon intermediate substrate, and ranging to a maximum value θ_(max) onthe opposite side of the layer, as in the first layer.

In another preferred embodiment of the present invention, the tilt anglein the second layer varies in the opposite direction of the first layer,i.e. when the first layer varies from θ_(min) to θ_(max), the secondvaries from θ_(max) to θ_(min), and vice versa.

In a preferred embodiment of the present invention, the minimum tiltangle θ_(min) in each layer is substantially zero degrees.

In another preferred embodiment of the present invention the opticalretardation film exhibits structure wherein the projection of theoptical symmetry axis of the first layer into the plane of the layer andthe projection of the optical symmetry axis of the second layer into theplane of the layer are twisted relative to each other at an angle ρ inthe plane of the interface between the layers, said angle ρ beingpreferably from 0 to 90 degrees.

In a preferred embodiment of the present invention the angle ρ issubstantially 0 degrees.

In another preferred embodiment of the present invention the retardationof the optical retardation film is from 50 to 250 nm.

Another object of the present invention is an optical retardation filmas described above that is obtainable by a method comprising thefollowing steps

-   A) coating a mixture comprising    -   a) a polymerizable mesogenic material comprising at least one        polymerizable mesogen having at least one polymerizable        functional group,

b) an initiator, and

-   -   c) optionally a solvent    -   on a substrate or between a first and a second substrate in form        of a layer,

-   B) aligning the polymerizable mesogenic material in the coated layer    into a tilted and optionally a splayed structure.

-   C) polymerizing said mixture of a polymerizable mesogenic material    by exposure to heat or actinic radiation,

-   D) optionally removing the substrate or, if two substrates are    present, one or two of the substrates from the polymerized material,    and

-   E) repeating the steps A), B), C) and optionally step D) at least    one more time.

In a preferred embodiment of the present invention the steps A), B) andC) are carried out on both sides of one substrate.

In another preferred embodiment of the present invention the mixture ofthe polymerizable mesogenic material used in the method described abovecomprises at least one polymerizable mesogen having one polymerizablefunctional group and at least one polymerizable mesogen having two ormore polymerizable functional groups.

In another preferred embodiment of the present invention thepolymerizable mesogens are compounds of formula IP-(Sp-X)_(n)-MG-R   Iwherein

-   P is a polymerizable group,-   Sp is a spacer group having 1 to 20 C atoms,-   X is a group selected from —O—, —S—, —CO—, —COO—, —OCO—, —OCOO— or a    single bond,-   n is 0 or 1,-   MG is a mesogenic or mesogenity supporting group, preferably    selected according to formula II    -(A¹-Z¹)_(m)-A²-Z²-A³-  II-    wherein    -   A¹, A² and A³ are independently from each other 1,4-phenylene in        which, in addition, one or more CH groups may be replaced by N,        1,4-cyclohexylene in which, in addition, one or two non-adjacent        CH₂ groups may be replaced by O and/or S, 1,4-cyclohexenylene or        naphthalene-2,6-diyl, it being possible for all these groups to        be unsubstituted, mono- or polysubstituted with halogen, cyano        or nitro groups or alkyl, alkoxy or alkanoyl groups having 1 to        7 C atoms wherein one or more H atoms may be substituted by F or        Cl,    -   Z¹ and Z² are each independently —COO—, —OCO—, —CH₂CH₂—, —OCH₂—,        —CH₂O—, —CH═CH—, —C≡C—, —CH═CH—COO—, —OCO—CH═CH— or a single        bond,    -   m is 0, 1 or 2, and    -   R is an alkyl radical with up to 25 C atoms which may be        unsubstituted, mono- or polysubstituted by halogen or CN, it        being also possible for one or more non-adjacent CH₂ groups to        be replaced, in each case independently from one another, by        —O—, —S—, —NH—, —N(CH₃)—, —CO—, —COO—, —OCO—, —OCO—O—, —S—CO—,        —CO—S— or —C≡C— in such a manner that oxygen atoms are not        linked directly to one another, or alternatively R is halogen,        cyano or has independently one of the meanings given for        P-(Sp-X)_(n)-.

Another object of the present invention is a means to producesubstantially linear polarized light, comprising a broadband circularreflective polarizer and an optical retardation film as described in theforegoing and the following.

Another object of the present invention is a liquid crystal displaycomprising a display cell and an optical retardation film as describedin the foregoing and the following.

Yet another object of the present invention is a liquid crystal displaycomprising a display cell and a means to produce substantially linearpolarized light as described in the foregoing and the following.

As mentioned above, a preferred embodiment of the present invention ischaracterized in that the optical retardation film exhibits a tilted andsplayed structure, wherein the tilt angle θ in each layer variescontinuously in a direction normal to the layer, starting from a minimumvalue θ_(min) at the side of the layer facing the other layer or, ifpresent, facing the common intermediate substrate, and ranging to amaximum value θ_(max) on the opposite side of the layer, or the otherway round.

FIG. 1 a depicts the structure of an optical retardation film 10according to this preferred embodiment in side view, as an example thatshould illustrate, but not limit, the scope of the present invention.The optical retardation film 10 is comprising a first layer 11 a and asecond layer 11 b of a polymerized mesogenic material on both sides of acommon intermediate substrate 12.

Each of the layers 11 a and 11 b shown in FIG. 1 a is exhibiting atilted and splayed structure, wherein the mesogens of the polymerizedmaterial are oriented such that the major optical axes in differentparts of the layer, of which locations at the surface are represented bylines 13 a/b and 16 a/b, and arbitrary intermediate locations arerepresented by lines 14 a/b and 15 a/b, each have a tilt angle θrelative to the plane of the layer, which is increasing in a directionnormal to the layer, beginning with a minimum value θ_(min) on the sideof each layer facing the common substrate 12.

In the inventive optical retardation film that is shown in FIG. 1 a, themesogens on the side of each layer facing the substrate have a planarorientation, i.e. the major optical axes of these mesogens, representedby lines 13 a and 13 b, have a minimum tilt angle θ_(min) that issubstantially 0 degrees. However, other values of θ_(min) are alsopossible.

The orientation of the optical axes in the two layers of the inventiveoptical retardation film depicted in FIG. 1 a can further be describedby an angle ρ in the plane of the common substrate 12 between the lines17 a and 17 b, wherein 17 a is representing the projection of the majoroptical axis in different parts of the first layer, indicated by lines13 a, 14 a, 15 a and 16 a, into the plane of the first layer, and 17 bis representing the projection of the major optical axis in differentparts of the second layer, indicated by lines 13 b, 14 b, 15 b and 16 b,into the plane of the second layer.

Thus, this angle ρ gives the amount by which the two layers are twistedagainst each other.

In the preferred embodiment shown in FIG. 1 a, the angle ρ between lines17 a and 17 b is substantially zero degrees. However, other values of ρare also possible.

FIG. 1 b illustrates the molecular structure of an optical retardationfilm 10 according to another preferred embodiment of the presentinvention. The numbered elements in FIG. 1 b have the same meaning asgiven for FIG. 1 a. The film 10 in FIG. 1 b consists of two layers 11 aand 11 b that are coated on two substrates 12 a and 12 b, wherein thetilt angle θ in each layer 11 a and 11 b varies continuously in adirection normal to the layer, beginning on the side of each layerfacing the common substrate 12 a between the layers with a minimum valueθ_(min) in the first layer 11 a and a maximum value θ_(max) in thesecond layer 11 b.

In an inventive optical retardation film according to the preferredembodiment which is exemplarily described in FIGS. 1 a and 1 b, thesense of variation of the tilt angle θ in each layer may be clockwise orcounterclockwise when going from low values to high values of θ, andwhen looking at the layer in side view. The sense of variation of thetilt angle θ can be the same or different in both layers. Preferably thesense of variation is opposite in the two layers, as for exampledepicted in FIGS. 1 a and 1 b.

In the optical retardation films according to the preferred embodimentwhich is described above, and which is examplarily depicted in FIGS. 1 aand 1 b, the minimum tilt angle θ_(min) is preferably from 0 to 20degrees, in particular from 0 to 10 degrees, most preferably from 0 to 5degrees.

The maximum tilt angle θ_(max) in an optical retardation film accordingto this preferred embodiment is preferably from 20 to 90 degrees, inparticular from 30 to 90 degrees, most preferably from 35 to 90 degrees.

Preferably the tilt angles of the two layers a and b at surfaces facingeach other either directly or via a common substrate are the same, thatis either θ_(min, a)=θ_(min, b) or θ_(max, a)=θ_(max, b). Verypreferably both the minimum and the maximum tilt angle of the two layersare the same, that is θ_(min, a)=θ_(min, b) and θ_(max, a)=θ_(max, b).

In another preferred embodiment of the present invention, the opticalretardation film is characterized in that the tilt angle θ issubstantially constant in each layer. The molecular structure of anoptical retardation film according to this preferred embodiment isexemplarily described in FIG. 1 c, wherein the numbered elements havethe same meanings as given for FIG. 1 a.

Thus, an optical retardation film according to this preferred embodimentcan be described by one tilt angle θ for each layer, which is rangingfrom 0 to 90 degrees, with the tilt angle in the first layer and thetilt angle in the second layer being independent of one another butpreferably having substantially the same value.

The tilt angle θ according to this preferred embodiment is preferablyfrom 5 to 80 degrees, in particular from 15 to 70 degrees, verypreferably from 25 to 60 degrees.

In the preferred embodiments as shown in FIGS. 1 a to 1 c, the angle ρbetween lines 17 a and 17 b is substantially zero degrees. However,other values for the angle ρ that are different from zero degrees arealso possible. In principle, the angle ρ can vary from 0 to 90 degrees.Preferably ρ is substantially 0 degrees or 90 degrees, most preferably 0degrees.

In case the tilt angles θ_(min) and θ_(max) of the two layers areidentical, that is θ_(min, a)=θ_(min, b) and θ_(max, a)=θ_(max, b), andthe angle ρ is substantially zero degrees, the two layers have astructure which in the ideal case the ideal case shows mirror symmetry,with the interface or, if present, the substrate between the layersbeing a mirror plane, like depicted in FIGS. 1 a and 1 c.

Preferably the inventive optical retardation film exhibits a symmetricalstructure wherein the interface or, if present, the substrate betweenthe two layers is a mirror plane.

FIG. 1 b represents an embodiment which is optically equivalent to theembodiment shown in FIG. 1 a, but with the two layers being coated ontwo separate substrates.

Analoguously the corresponding optically equivalent to the embodimentshown in FIG. 1 c can be obtained by combining two layers with constanttilt angle formed on two separate substrates.

The inventive optical retardation film does not necessarily have tocomprise a substrate between the two layers of anisotropic polymer. In apreferred embodiment of the present invention, the two layers aredirectly adjacent to each other. Such an optical retardation film can bedescribed in analogy to FIGS. 1 a to 1 c, wherein 12 and 12 arespectively are indicating the interface between the two layers.

The retardation of the inventive optical retardation film is preferablyranging from 50 to 250 nm, very preferably from 60 to 200 nm, mostpreferably from 70 to 170 nm.

Another object of the present invention is a means to producesubstantially linear polarized light, comprising an optical retardationfilm as described in the foregoing and the following in combination witha broadband circular reflective polarizer. When using this combination,light that is substantially linearly polarized can be produced.

The bandwidth of the wavelength band reflected by said broadbandcircular reflective polarizer is at least 100 nm, preferably at least150 nm, most preferably at least 200 nm, ideally 250 nm or larger.Preferably the bandwidth of the circular reflective polarizer iscovering the spectrum of visible light.

In another preferred embodiment of the present invention, theretardation of the optical retardation film is substantially 0.25 timesa wavelength reflected by the broadband circular reflective polarizer,so that the optical retardation film serves as a quarter waveretardation film.

For a liquid crystal display comprising a broad band circular reflectivepolarizer and an optical retardation film of the state of the art, likee.g. a quarter wave film (QWF) made of stretched PVA, the luminance atnormal incidence (viewing angle 0°) and at low values of the viewingangle is increased compared to a liquid crystal display comprising thecircular reflective polarizer alone without an optical retardation film.

However, as the display comprising the a broad band circular reflectivepolarizer and a state of the art QWF as mentioned above is viewed underan increasing angle, the increasing phase retardation by the QWF itselfcauses a reduction to the luminance, coinciding with the value measuredfor the display comprising the circular reflective polarizer as a singlecomponent at a certain angle. This angle is referred to as the‘cross-over angle’ α_(c).

When using an inventive optical retardation film instead of a state ofthe art QWF in the liquid crystal display, the cross-over angle α_(c)can be increased significantly. In other words, the brightnessenhancement, i.e. the increase of luminance at low viewing angles, thatwas achieved by using the circular reflective polarizer can be extendedalso to large viewing angles.

The cross over angle α_(c) of a display comprising a combination ofoptical elements comprising an optical retardation film and a broadbandcircular reflective polarizer according to the present invention ispreferably 25° or larger, particularly preferably 30° or larger, veryparticularly preferably 35° or larger in at least one direction ofobservation, e.g. in horizontal or vertical direction.

The means to produce substantially linear polarized light can compriseother optical elements in addition to the inventive optical retardationfilm and the circular reflective polarizer. Preferably said meansadditionally comprises at least one of the following elements:

-   I) a compensation film comprising a layer of an anisotropic polymer    material with a homeotropic or tilted homeotropic orientation, said    compensation film being positioned adjacent to either side of the    inventive optical retardation film,-   II) a linear polarizer, arranged in such a manner that the inventive    optical retardation film and, if present, the compensation film I)    are positioned between the broadband circular reflective polarizer    and the linear polarizer,-   III) a radiation source, and-   IV) a diffusor adjacent to the radiation source,    wherein the components I to IV are arranged in such a manner that    the broadband circular reflective polarizer of the combination of    optical elements is facing the radiation source III or, if present,    the diffusor IV.

As a linear polarizer II a commercially available polarizer can be used.In a preferred embodiment of the present invention the linear polarizeris a low contrast polarizer. In another preferred embodiment of thepresent invention the linear polarizer II is a dichroic polarizer.

As a radiation source III preferably a standard backlight for liquidcrystal displays, like e.g. a side-lit or a meander type backlight, isused. These backlights typically comprise a lamp, a reflector, a lightguide and optionally a diffuser.

The radiation source III can also consist of a reflector that reflectsradiation generated outside the means to produce substantially linearpolarized light. The display device according to the present inventioncan then be used as a reflective display.

The means to produce substantially linear polarized light according tothe present invention can further comprise

-   V) one or more adhesive layers provided to at least one of the    optical components comprising the circular reflective polarizer, the    inventive optical retardation film or one of the components I to IV    described above,-   VI) one or more protective layers provided to at least one of the    optical components comprising the circular reflective polarizer, the    inventive optical retardation film or one of the components I to V    described above.

Another object of the present invention is a liquid crystal displaycomprising a display cell and a means to produce substantially linearpolarized light as described in the foregoing and the following.

The function of the inventive means to produce substantially linearpolarized light is further explained by FIG. 2, which shows a displaydevice according to a preferred embodiment of the present invention, asan example that should not limit the scope of the invention. The maindirection of light following the optical path is from the left side tothe right side. The display device 20 consists of a side-lit backlightunit 21 with a lamp 22 a and a combined light guide and reflector 22 b,a diffusor 23 and a polarizer combination consisting of a circularreflective polarizer 24 comprising a layer of a liquid crystallinematerial with a helically twisted molecular orientation, the inventiveoptical retardation film 25, a compensation film 26 and a linearpolarizer 27. The figure further depicts a liquid crystal cell 28 and asecond linear polarizer 29 behind the display cell.

Light emitted from the backlight 21 is interacting with the molecularhelix structure of the circular reflective polarizer 24 with the resultthat 50% of the intensity of the light incident on the circularreflective polarizer is reflected as circular polarized light of thesame twist sense as that of the molecular helix structure of thecircular reflective polarizer, which may be either right-handed orleft-handed, whereas the other 50% of the incident light are reflectedas circular polarized light of the opposite twis sense. The reflectedlight is depolarized in the backlight and redirected by the reflector 22b onto the circular reflective polarizer 24. In this manner,theoretically 100% of the light of a broad range of wavelengths emittedfrom the backlight 21 are converted into circularly polarized light.

The main part of the transmitted component is converted by the inventiveoptical retardation film 25 into linear polarized light, which is thencompensated by the compensation film 26 and being transmitted by thelinear polarizer 27, whereas light which is not completely transferredinto linear polarized light by the optical retardation film 25, such aselliptically polarized light, is not transmitted by the linear polarizer27. The linear polarized light then passes through the display 28 andthe second linear polarizer 29 to reach the viewer 30.

FIG. 2 b depicts a display device according to another preferredembodiment of the invention having essentially the same assembly as thatshown in FIG. 1 a, with the difference that in the assembly shown inFIG. 2 b the inventive optical retardation film 25 is placed behind thecompensation film 26 when looking from the direction of incident light.

As mentioned above, it is also possible to use the inventive opticalretardation film as a compensation film for conventional displays, likeTN, STN, AMD-TN or other types of displays.

Thus, another object of the present invention is a liquid crystaldisplay comprising a display cell and an optical retardation film asdescribed in the foregoing and the following.

Preferably the optical retardation film is obtainable by a processcomprising the following steps

-   A) coating a mixture comprising    -   a) a polymerizable mesogenic material comprising at least one        polymerizable mesogen having at least one polymerizable        functional group,    -   b) an initiator, and    -   c) optionally a solvent    -   on a substrate or between a first and a second substrate in form        of a layer,-   B) aligning the polymerizable mesogenic material in the coated layer    into a tilted and optionally a splayed structure.-   C) polymerizing said mixture of a polymerizable mesogenic material    by exposure to heat or actinic radiation,-   D) optionally removing the substrate or, if two substrates are    present, one or two of the substrates from the polymerized material,    and-   E) repeating the steps A), B), C) and optionally step D) at least    one more time.

In a preferred embodiment of the present invention the steps A), B) andC) are carried out on both sides of a common substrate. In this way anoptical retardation film as described in FIG. 1 is obtained.

As a substrate for example a glass or quarz sheet as well as a plasticfilm or sheet can be used. It is also possible to put a second substrateon top of the coated mixture prior to and/or during and/or afterpolymerization. The substrates can be removed after polymerization ornot. When using two substrates in case of curing by actinic radiation,at least one substrate has to be transmissive for the actinic radiationused for the polymerization.

Isotropic or birefringent substrates can be used. In case the substrateis not removed from the polymerized film after polymerization,preferably isotropic substrates are used.

Preferably at least one substrate is a plastic substrate such as forexample a film of polyester such as polyethyleneterephthalate (PET), ofpolyvinylalcohol (PVA), polycarbonate (PC) or triacetylcellulose (TAC),especially preferably a PET film or a TAC film. As a birefringentsubstrate for example an uniaxially stretched plastic film can be used.For example PET films are commercially available from ICI Corp. underthe trade name Melinex.

The mixture comprising the polymerizable mesogenic material canadditionally comprise a solvent. If the mixture contains a solvent, thisis preferably evaporated after coating the mixture onto the substratebefore polymerization. In most cases it is suitable to heat the mixturein order to facilitate the evaporation of the solvent.

In principal every solvent can be used that is known to the skilled inthe art for this purpose. Preferably a solvent is used wherein thepolymerizable mesogenic material dissolves easily. Typically an organicsolvent, like e.g. toluene, is used.

In another preferred embodiment of the present invention as describedabove, each polymerized layer of the mesogenic material on the sidefacing the other layer or the substrate exhibits substantially planaralignment, i.e. the minimum tilt angle θ_(min) is substantially zerodegrees. Planar alignment can be achieved for example by shearing thematerial, e.g. by means of a doctor blade. It is also possible to applyan alignment layer, for example a layer of rubbed polyimide or sputteredSiO_(x), on top of at least one of the substrates.

An especially preferred embodiment of the present invention ischaracterized in that planar alignment of the polymerizable mesogenicmaterial is achieved by directly rubbing the substrate, i.e. withoutapplying an additional alignment layer. This is a considerable advantageas it allows a significant reduction of the production costs of theoptical retardation film. In this way a low tilt angle can easily beachieved.

Preferably a plastic film, in particular a polyester film, e.g. Melinex,or a TAC film are used as a substrate in this preferred embodiment.

For example rubbing can be achieved by means of a rubbing cloth or witha flat bar coated with a rubbing cloth.

In another preferred embodiment of the present invention rubbing isachieved by means of a at least one rubbing roller, like e.g. a fastspinning roller that is brushing over the substrate, or by putting thesubstrate between at least two rollers, wherein in each case at leastone of the rollers is optionally covered with a rubbing cloth.

In another preferred embodiment of the present invention rubbing isachieved by wrapping the substrate at least partially at a defined anglearound a roller that is preferably coated with a rubbing cloth.

As rubbing cloth all materials can be used that are known to the skilledin the art for this purpose. For example velvet of a commerciallyavailable standard type can be used as a rubbing cloth.

Preferably rubbing is carried out only in one direction. Very preferablythe rubbing direction on both sides of the substrate is the same.

A preferred embodiment of the present invention is characterized in thatthe two layers of the mesogenic material are adjacent to both sides of acommon substrate, like e.g. depicted in FIG. 1 a.

Such an optical retardation film, can be obtained for example bycoating, aligning and polymerizing a mesogenic material on a substratethat has been rubbed unidirectionally on one side, and subsequentlyrepeating the same process on the other side of the same substrate,wherein the substrate in the second process is rubbed in the samedirection as in the first process.

Such an optical retardation film is advantageous compared to its opticalequivalent, like for example shown in FIG. 1 b, in that it is thinnerand requires a lower amount of substrate.

An optical retardation film with a angle ρ different from 0 degrees asdescribed above can be obtained if the rubbing directions on the twosurfaces of the substrate are different from each other.

When using an anisotropic substrate, rubbing is preferably carried outunidirectionally in a direction substantially parallel to the majorsymmetry axis of the substrate.

The ability of the substrate to induce alignment in an inventivepolymerizable mesogenic composition, which is coated on this substrateafter rubbing the substrate, will depend on the process parameters ofthe rubbing process, like the rubbing length, the rubbing pressure andrubbing speed and, in case a rubbing roller is used, on the rotationalvelocity of the roller, the rubbing roller circumference and the tensionon the substrate.

Thus, the rubbing length in the rubbing process according to thepreferred embodiments described above is also depending on the otherpreviously described process parameters of the rubbing process.Preferably the rubbing length is from 0.2 to 5 meters, in particularfrom 0.5 to 3 meters.

The orientation of the mesogenic material depends, inter alia, on thefilm thickness, the type of substrate, the alignment state of thesubstrate surface, and the composition of the polymerizable mesogenicmaterial. Thus, by changing these parameters, it is possible to controlthe structure of the optical retardation film, and in particular thestructure parameters like e.g. the tilt angle and its degree ofvariation.

When changing the composition of the polymerizable mesogenic mixture,for example by varying the ratio of direactive to monoreactive compoundsand/or polar to unpolar compounds, the alignment profile in thedirection perpendicular to the film plane can be altered.

In a preferred embodiment of the present invention the opticalretardation film is obtained by a process as described above, whereinthe substrate is removed from the polymerized mesogenic material afterthe first polymerization step. The second layer of polymerizablemesogenic material is then coated directly on the polymerized firstlayer, which acts as a substrate, and is then aligned and polymerized.In this way an optical retardation film is obtained that comprises twolayers of polymerized mesogenic material that are directly adjacent toeach other.

When preparing an optical retardation film according to this preferredembodiment, in some cases it is even not necessary to apply specialalignment means or methods, since the first polymerized layer can act asaligning layer and can induce alignment in the second layer.

Polymerization of the inventive polymerizable mesogenic mixture takesplace by exposing it to heat or to actinic radiation. Actinic radiationmeans irradiation with light, X-rays, gamma rays or irradiation withhigh energy particles, such as ions or electrons. In particularpreferably UV light is used. The irradiation wavelength is preferablyfrom 220 nm to 420 nm.

As a source for actinic radiation for example a single UV lamp or a setof UV lamps can be used. When using a high lamp power the curing timecan be reduced.

The curing time is dependening, inter alia, on the reactivity of thepolymerizable mesogenic material, the thickness of the coated layer, thetype of polymerization initiator and the power of the UV lamp. For massproduction short curing times are preferred. The curing time accordingto the invention is preferably not longer than 30 minutes, especiallypreferably not longer than 15 minutes and very particularly preferablyshorter than 8 minutes.

The polymerization is carried out in the presence of an initiatorabsorbing the wavelength of the actinic radiation. For example, whenpolymerizing by means of UV light, a photoinitiator can be used thatdecomposes under UV irradiation to produce free radicals that start thepolymerization reaction. As a photoinitiator for radicalicpolymerization a commercially available photoinitiator like e.g.Irgacure 651 (by Ciba Geigy AG, Basel, Switzerland) can be used.

It is also possible to use a cationic photoinitiator, when curingpolymerizable mesogens with for example vinyl and epoxide polymerizablegroups, that photocures with cations instead of free radicals. Thepolymerization may also be started by an initiator that initiates thepolymerization when heated above a certain temperature.

In addition to light- or temperature-sensitive initiators thepolymerizable mixture may also comprise one or more other suitablecomponents such as, for example, catalysts, stabilizers, co-reactingmonomers or surface-active compounds.

In a preferred embodiment of the invention, the polymerizable mixturecomprises a stabilizer that is used to prevent undesired spontaneouspolymerization for example during storage of the mixture. As stabilizersin principal all compounds can be used that are known to the skilled inthe art for this purpose. These compounds are commercially available ina broad variety. Typical examples for stabilizers are 4-ethoxyphenol orbutylated hydroxytoluene (BHT).

The polymerizable mixture according to this preferred embodimentpreferably comprises a stabilizer as described above at an amount of 1to 1000 ppm, especially preferably 10 to 500 ppm.

Other additives, like e.g. chain transfer agents, can also be added tothe polymerizable mixture in order to modify the physical properties ofthe inventive polymer film. For example when adding a chain transferagent to the polymerizable mixture, the length of the free polymerchains and/or the length of the polymer chains between two crosslinks inthe inventive polymer film can be controlled. When the amount of thechain transfer agent is increased, polymer films with decreasing polymerchain length are obtained.

In a preferred embodiment of the present invention the polymerizablemixture comprises 0.01 to 10%, in particular 0.1 to 5%, very preferably0.5 to 3% of a chain transfer agent. The polymer films according to thispreferred embodiment show especially good adhesion to a substrate, inparticular to a plastic film, like e.g. a TAC film.

As a chain transfer agent for example monofunctional thiol compoundslike e.g. dodecane thiol or multifunctional thiol compounds like e.g.trimethylpropane tri(3-mercaptopropionate) can be used.

In some cases it is of advantage to apply a second substrate to aidalignment and exclude oxygen that may inhibit the polymerization.Alternatively the curing can be carried out under an atmosphere of inertgas. However, curing in air is also possible using suitablephotoinitiators and high UV lamp power. When using a cationicphotoinitiator oxygen exclusion most often is not needed, but watershould be excluded. In a preferred embodiment of the invention thepolymerization of the polymerizable mesogenic material is carried outunder an atmosphere of inert gas, preferably under a nitrogenatmosphere.

To obtain polymer films with good alignment the polymerization has to becarried out in the liquid crystal phase of the polymerizable mesogenicmixture. Preferably polymerizable mesogenic mixtures are used that havea low melting point, preferably a melting point of 100° C. or lower, inparticular 60° C. or lower, so that curing can be carried out in theliquid crystalline phase of the mixture at low temperatures. This issimplifying the polymerization process as less heating of the mixture isrequired and there is less strain of the mesogenic materials, thesubstrates and the production equipment during polymerization, which isof importance especially for mass production. Curing temperatures below100° C. are preferred. Especially preferred are curing temperaturesbelow 60° C.

The thickness of the inventive optical retardation film obtained by themethod as described above is preferably from 0.1 to 10 μm, in particularfrom 0.2 to 5 μm, most preferably from 0.4 to 2 μm. For someapplications, a film thickness between 2 and 15 μm is also suitable.

In a preferred embodiment of the present invention, the broadbandcircular reflective polarizer and/or the compensation film of theinventive means to produce linear polarized light are comprising a layerof an anisotropic polymer material that is obtained by polymerizing anoriented layer of polymerizable mesogens.

Particularly preferably these polymerizable mesogens have a structurewhich is similar to that of the polymerizable mesogenic compounds offormula I as described above and below.

Thus, when using an inventive optical retardation film together with abroadband circular reflective polarizer and/or a compensation filmaccording to this preferred embodiment, it is possible to adapt theoptical properties of the optical retardation film to those of thecircular reflective polarizer and/or the compensation film by usingmaterials comprising compounds with similar structure. In this way acombination of an optical retardation film and a circular reflectivepolarizer and/or a compensation film with superior optical performancecan be obtained.

In a preferred embodiment the polymerizable mixture comprisespolymerizable mesogenic compounds having two or more polymerizablefunctional groups (referred to as di-/multireactive ordi-/multifunctional compounds). Upon polymerization of such a mixture athree-dimensional polymer network is formed. An optical retardation filmmade of such a network is self-supporting and shows a high mechanicaland thermal stability and a low temperature dependence of its physicaland optical properties.

In another preferred embodiment the polymerizable mixture comprises 0 to20% of a non mesogenic compound with two or more polymerizablefunctional groups to increase crosslinking of the polymer. Typicalexamples for difunctional non mesogenic monomers are alkyldiacrylates oralkyldimethacrylates with alkyl groups of 1 to 20 C atoms. Typicalexamples for non mesogenic monomers with more than two polymerizablegroups are trimethylpropanetrimethacrylate orpentaerythritoltetraacrylate.

By varying the concentration of the multifunctional mesogenic or nonmesogenic compounds the crosslink density of the polymer film andthereby its physical and chemical properties such as the glasstransition temperature, which is also important for the temperaturedependence of the optical properties of the optical retardation film,the thermal and mechanical stability or the solvent resistance can betuned easily.

The terms polymerizable or reactive mesogen, polymerizable or reactivemesogenic compound, polymerizable or reactive liquid crystal (compound)and polymerizable or reactive liquid crystalline compound as used in theforegoing and the following comprise compounds with a rodlike, boardlikeor disklike mesogenic group and at least one polymerizable functionalgroup. These mesogenic compounds do not necessarily have to exhibitmesophase behaviour by themselves. It is also possible that they showmesophase behaviour in mixtures with other compounds or afterpolymerization of the pure mesogenic compounds or of the mixturescomprising the mesogenic compounds.

In a particularly preferred embodiment of the present invention, thepolymerizable mesogens comprised by the mixture of the polymerizablemesogenic material are compounds of formula IP-(Sp-X)_(n)-MG-R   Iwherein

-   P is a polymerizable group,-   Sp is a spacer group having 1 to 20 C atoms,-   X is a group selected from —O—, —S—, —CO—, —COO—, —OCO—, —OCOO— or a    single bond,-   n is 0 or 1,-   MG is a mesogenic or mesogenity supporting group, preferably    selected according to formula II    -(A¹-Z¹)_(m)-A²-Z²-A³-  II    -   wherein    -   A¹, A² and A³ are independently from each other 1,4-phenylene in        which, in addition, one or more CH groups may be replaced by N,        1,4-cyclohexylene in which, in addition, one or two non-adjacent        CH₂ groups may be replaced by O and/or S, 1,4-cyclohexenylene or        naphthalene-2,6-diyl, it being possible for all these groups to        be unsubstituted, mono- or polysubstituted with halogen, cyano        or nitro groups or alkyl, alkoxy or alkanoyl groups having 1 to        7 C atoms wherein one or more H atoms may be substituted by F or        Cl,    -   Z¹ and Z² are each independently —COO—, —OCO—, —CH₂CH₂—, —OCH₂—,        —CH₂O—, —CH═CH—, —C—C—, —CH═CH—COO—, —OCO—CH═CH— or a single        bond and    -   m is 0, 1 or 2, and-   R is an alkyl radical with up to 25 C atoms which may be    unsubstituted, mono- or polysubstituted by halogen or CN, it being    also possible for one or more non-adjacent CH₂ groups to be    replaced, in each case independently from one another, by —O—, —S—,    —NH—, —N(CH₃)—, —CO—, —COO— —OCO—, —OCO—O—, —S—CO—, —CO—S— or —C≡C—    in such a manner that oxygen atoms are not linked directly to one    another, or alternatively R is halogen, cyano or has independently    one of the meanings given for P-(Sp-X)_(n)—.

Particularly preferred are polymerizable mixtures comprising at leasttwo polymerizable mesogenic compounds at least one of which is acompound of formula I.

In another preferred embodiment of the invention the polymerizablemesogenic compounds are selected according to formula I, wherein R hasone of the meanings of P-(Sp-X)_(n)— as given above.

Bicyclic and tricyclic mesogenic compounds are preferred.

Of the compounds of formula I especially preferred are those in which Ris F, Cl, cyano, or optionally halogenated alkyl or alkoxy, or has themeaning given for P-(Sp-X)_(n)-, and MG is of formula II wherein Z¹ andZ² are —COO—, —OCO—, —CH₂—CH₂—, —CH═CH—COO—, —OCO—CH═CH— or a singlebond.

A smaller group of preferred mesogenic groups of formula II is listedbelow. For reasons of simplicity, Phe in these groups is 1,4-phenylene,Phe L is a 1,4-phenylene group which is substituted by at least onegroup L, with L being F, Cl, CN or an optionally fluorinated alkyl,alkoxy or alkanoyl group with 1 to 4 C atoms, and Cyc is1,4-cyclohexylene.-Phe-Z²-Phe-  II-1-Phe-Z²-Cyc-  II-2-PheL-Z²-Phe-  II-3-PheL-Z²-Cyc-  II-4-Phe-Z²-PheL-  II-5-Phe-Z¹-Phe-Phe-  II-6-Phe-Z¹-Phe-Cyc-  II-7-Phe-Z¹-Phe-Z²-Phe-  II-8-Phe-Z¹-Phe-Z²-Cyc-  II-9-Phe-Z¹-Cyc-Z²-Phe-  II-10-Phe-Z¹-Cyc-Z²-Cyc-  II-11-Phe-Z¹-PheL-Z²-Phe-  II-12-Phe-Z¹-PheL-Z²-PheL-  II-13-PheL-Z¹-Phe-Z²-PheL-  II-14-PheL-Z¹-PheL-Z²-Phe-  II-15-PheL-Z¹-PheL-Z²-PheL-  II-16

In these preferred groups Z¹ and Z² have the meaning given in formula Idescribed above. Preferably Z¹ and Z² are —COO—, —OCO—, —CH₂CH₂—,—CH═CH—COO— or a single bond.

L is preferably F, Cl, CN, NO₂, CH₃, C₂H₅, OCH₃, OC₂H₅, COCH₃, COC₂H₅,CF₃, OCF₃, OCHF₂, OC₂F₅, in particular F, Cl, CN, CH₃, C₂H₅, OCH₃, COCH₃and OCF₃, most preferably F, CH₃, OCH₃ and COCH₃.

Particularly preferred are compounds wherein MG is selected from thefollowing formulae

wherein L has the meaning given above and r is 0, 1 or 2.

The group

in this preferred formulae is very preferably denoting

furthermore

with L having each independently one of the meanings given above.

R in these preferred compounds is particularly preferably CN, F, Cl,OCF₃, or an alkyl or alkoxy group with 1 to 12 C atoms or has one of themeanings given for P-(Sp-X)_(n)—.

If R in formula I is an alkyl or alkoxy radical, i.e. where the terminalCH₂ group is replaced by —O—, this may be straight-chain or branched. Itis preferably straight-chain, has 2, 3, 4, 5, 6, 7 or 8 carbon atoms andaccordingly is preferably ethyl, propyl, butyl, pentyl, hexyl, heptyl,octyl, ethoxy, propoxy, butoxy, pentoxy, hexoxy, heptoxy, or octoxy,furthermore methyl, nonyl, decyl, undecyl, dodecyl, tridecyl,tetradecyl, pentadecyl, methoxy, nonoxy, decoxy, undecoxy, dodecoxy,tridecoxy or tetradecoxy, for example.

Oxaalkyl, i.e. where one CH₂ group is replaced by —O—, is preferablystraight-chain 2-oxapropyl (=methoxymethyl), 2-(=ethoxymethyl) or3-oxabutyl (=2-methoxyethyl), 2-, 3-, or 4-oxapentyl, 2-, 3-, 4-, or5-oxahexyl, 2-, 3-, 4-, 5-, or 6-oxaheptyl, 2-, 3-, 4-, 5-, 6- or7-oxaoctyl, 2-, 3-, 4-, 5-, 6-, 7- or 8-oxanonyl or 2-, 3-, 4-, 5-,6-,7-, 8- or 9-oxadecyl, for example.

In addition, mesogenic compounds of the formula I containing a branchedgroup R can be of importance as comonomers, for example, as they reducethe tendency towards crystallization. Branched groups of this typegenerally do not contain more than one chain branch. Preferred branchedgroups are isopropyl, isobutyl (=methylpropyl), isopentyl(=3-methylbutyl), isopropoxy, 2-methylpropoxy and 3-methylbutoxy.

P in formula I is preferably selected form CH₂═CW—COO—, WCH═CH—O—,

or CH₂═CH-Phenyl-(O)_(k)— with W being H, CH₃ or Cl and k being 0 or 1,

P is particularly preferably a vinyl group, an acrylate group, amethacrylate group, a propenyl ether group or an epoxy group, veryparticularly preferably an acrylate or methacrylate group.

As for the spacer group Sp in formula I, IIa and Ib all groups can beused that are known for this purpose to the skilled in the art. Thespacer group Sp is preferably linked to the polymerizable group P by anester or ether group or a single bond. The spacer group Sp is preferablya linear or branched alkylene group having 1 to 20 C atoms, inparticular 1 to 12 C atoms, in which, in addition, one or morenon-adjacent CH₂ groups may be replaced by —O—, —S—, —NH—, —N(CH₃)—,—CO—, —O——CO—, —S—CO—, —O—COO—, —CO—S—, —CO—O—, —CH(halogen)—, —CH(CN)—,—CH═CH— or —C≡C—.

Typical spacer groups Sp are for example —(CH₂)_(o)—,—(CH₂CH₂O)_(r)—CH₂CH₂—, —CH₂CH₂—S—CH₂CH₂— or —CH₂CH₂—NH—CH₂CH₂—, with obeing an integer from 2 to 12 and r being an integer from 1 to 3.

Preferred spacer groups Sp are ethylene, propylene, butylene, pentylene,hexylene, heptylene, octylene, nonylene, decylene, undecylene,dodecylene, octadecylene, ethyleneoxyethylene, methyleneoxybutylene,ethylene-thioethylene, ethylene-N-methyl-iminoethylene and1-methylalkylene, for example.

In the event that R is a group of formula P-Sp-X— or P-Sp- respectively,the spacer groups on each side of the mesogenic core may be identical ordifferent.

In particular preferred are compounds of formula I wherein n is 1.

In another preferred embodiment, the inventive compensator is obtainedby copolymerizing mixtures comprising compounds of formula I wherein nis 0 and compounds of formula I wherein n is 1.

Typical examples representing polymerizable mesogenic compounds of theformula I can be found in WO 93/22397; EP 0 261 712; DE 195 04 224; DE44 08 171 or DE 44 05 316. The compounds disclosed in these documents,however are to be regarded merely as examples that should not limit thescope of this invention.

Furthermore, typical examples representing polymerizable mesogeniccompounds are shown in the following list of compounds, which is,however, to be understood only as illustrative without limiting thescope of the present invention:

In these compounds x and y are each independently 1 to 12, A is a1,4-phenylene or 1,4-cyclohexylene group, R¹ is halogen, cyano or anoptionally halogenated alkyl or alkoxy group with 1 to 12 C atoms and L¹and L² are each independently H, F, Cl, CN, or an optionally halogenatedalkyl, alkoxy or alkanoyl group with 1 to 7 C atoms.

The polymerizable mesogenic compounds disclosed in the foregoing and thefollowing can be prepared by methods which are known per se and whichare described in the documents cited above and, for example, in standardworks of organic chemistry such as, for example, Houben-Weyl, Methodender organischen Chemie, Thieme-Verlag, Stuttgart.

As mentioned above, the properties of an inventive optical retardationfilm, like e.g. mesogen orientation and film structure, temperaturestability and optical performance, can easily be adjusted to the desiredspecification by varying the composition of the polymerizable mesogenicmaterial.

One possible way to adjust the alignment profile in the directionperpendicular to the film plane is the appropriate selection of theratio of monoreactive mesogenic compounds, i.e. compounds with onepolymerizable group, and direactive mesogenic compounds, i.e.—compoundswith two polymerizable groups.

For a highly splayed film structure, preferably the ratio of mono- todireactive mesogenic compounds should be in the range of 6:1 to 1:2,preferably 3:1 to 1:1, especially preferably about 3:2.

Another effective means to adjust the desired splay geometry is to use adefined amount of dielectrically polar reactive mesogens in thepolymerizable mesogenic mixture. These polar reactive mesogens can beeither monoreactive or direactive. They can be either dielectricallypositive or negative. Most preferred are dielectrically positive andmonoreactive mesogenic compounds.

The amount of the polar compounds in the polymerizable mesogenic mixtureis preferably 1 to 80%, especially 3 to 60%, in particular 5 to 40% byweight of the total mixture.

Polar compounds bear one or more polar groups. Preferably these groupsare selected from terminal or lateral end groups like CN, F, Cl, OCF₃,OCF₂H, OC₂F₅, CF₃, OCN or SCN, or from bridge groups like —COO—, —OCO—,—O—, —S—, —OCH₂—, —CH₂O—, —OCOO—, —COO—CH═CH— or —CF₂═CF₂—. Verypreferably these groups are selected from CN, F, Cl and OCF₃.

Furthermore, these polar compounds preferably have a high absolute valueof the dielectric anisotropy Δ∈, which is typically higher than 1.5.Thus, dielectrically positive compounds preferably exhibit Δ∈>1.5 anddielectrically negative polar compounds preferably exhibit Δ∈<−1.5. Verypreferred are dielectrically positive polar compounds with Δ∈>3, inparticular with Δ∈>5.

In a preferred embodiment of the present invention, the opticalretardation film is obtainable from a mixture of a polymerizablemesogenic material comprising the following components

-   a1) 10 to 99% by weight of at least one mesogen according to formula    I having one polymerizable functional group,-   a2) 0 to 70% by weight of at least one mesogen according to formula    I having two or more polymerizable functional groups,-   b) 0.01 to 5% by weight of an initiator,

Mixtures according to this preferred embodiment are preferred thatcomprise

-   a1A) 10to 65%, preferably 15 to 50% by weight of at least one    polymerizable mesogen of formula I having one polymerizable group,    wherein R is an alkyl or alkoxy group with 1 to 12 C atoms,-   a1B) 5 to 40%, preferably 8 to 30% by weight of at least one    polymerizable mesogen of formula I having one polymerizable group,    wherein R is CN, F, Cl or a halogenated alkyl or alkoxy group with 1    to 12 C atoms,-   a2) 2 to 90%, preferably 5 to 60% by weight of at least one    polymerizable mesogen of formula I having two polymerizable groups,    wherein R has one of the meanings of P-(Sp-X—)_(n).-   b) 0.01 to 5% by weight of an initiator.

Very preferred are mixtures according to this particularly preferredembodiment wherein the ratio of components a1A to a1B to a2 is in therange of 2:1:2.

Further preferred are mixtures according to this particularly preferredembodiment that comprise two to eight, in particular two to six, mostpreferably two to four different polymerizable mesogens of componentsa1A and a1B.

In the mixtures comprising two or more different mesogens according toformula I having one polymerizable functional group as described above,preferably each of the different mesogens according to formula I isdifferent in at least one of the groups P, Sp, X, A¹, A², A³, Z¹, Z² andR from each other of the mesogens.

Furthermore, it has been found that when using monoreactive unpolarcompounds, like e.g. compounds of formula Ia

-   -   wherein x is 1 to 12, R² is C₁₋₁₂ alkyl or alkoxy, and    -   A⁴ is denoting 1,4-phenylene, trans-1,4-cyclohexylene or a        single bond, preferably 1,4-cyclohexylene,        in a defined amount, the alignment profile and the tilt angle of        the inventive optical retardation film can easily be adjusted to        the desired specification.

For example, when preparing an optical retardation film as describedabove from a polymerizable mesogenic mixture comprising monoreactiveunpolar compounds, e.g. compounds of formula la, together withdireactive compounds and dielectrically positive polar monoreactivecompounds of formula I, the tilt angle of the film is increasing withincreasing amount of the monoreactive unpolar compounds.

In a preferred embodiment of the present invention, the polymerizablemesogenic mixture comprises 2 to 30%, preferably 5 to 25% by weight ofcompounds of formula Ia.

In another preferred embodiment, the polymerizable mesogenic mixturecomprises 15 to 60%, preferably 20 to 55% by weight of compounds offormula Ia.

In another preferred embodiment, the polymerizable mesogenic mixturecomprises 45 to 90%, preferably 50 to 85% by weight of compounds offormula Ia.

Without further elaboration one skilled in the art can, using thepreceding description, utilize the present invention to its fullestextent. The following examples are, therefore, to be construed as merelyillustrative and not limitative of the remainder of the disclosure inany way whatsoever.

EXAMPLES

In the foregoing and in the following examples, unless otherwiseindicated, all temperatures are set forth uncorrected in degrees Celsiusand all parts and percentages are by weight. The following abbreviationsare used to illustrate the liquid crystalline phase behaviour of thecompounds:

K=crystalline; N=nematic; S=smectic; Ch=cholesteric; I=isotropic. Thenumbers between these symbols indicate the phase transition temperaturesin degree Celsius.

Example 1

The following polymerizable mixture was formulated

-   compound (1) 38.9%-   compound (2) 38.9%-   compound (3) 19.2%-   Irgacure 907 3.0%    Irgacure 907 is a commercially available photoinitiator (Ciba Geigy    AG) of the following formula

In order to prepare an optical retardation film, a solution of 12% byweight of the above given polymerizable mixture in toluene is coatedonto a TAC film (Lonza triphan type 91) that has been rubbed with avelvet coated bar unidirectionally at a length of 500 mm. The thicknessof the coated solution is 6 μm. The toluene is evaporated to give a 0.7μm thick layer of the above given polymerizable mixture. The layer isthen polymerized by irradiation with UV light of 254 nm with anirradiance of 1500 mW/cm².

The rubbing, coating and curing process is then repeated on the otherside of the TAC film, wherein the rubbing of the TAC substrate iscarried out in the same direction as in the first step. A polymer filmis obtained that can be used as an optical retardation film.

Use Example A

The retardation of the optical retardation film of example 1 wasmeasured using an Olympus BX-50 polarizing microscope equipped with aquartz Berek type compensator and a tilting (−60° to +60°) and rotating(0 to 360°) stage.

The retardation of the optical retardation film of example 1 is 100 nm,measured at a wavelength of 550 nm and normal incidence.

To compare the performance of the inventive optical retardation filmwith retardation films of prior art, the measurement described above wasrepeated with the following retardation films:

-   1) a retardation film consisting of stretched polyvinylalcohol (PVA)    available from Polatechno (17140 T),-   2) a retardation film consisting of stretched polycarbonate (PC),-   3) a retardation film consisting of a polymerized mesogenic material    with planar alignment, comprising compounds similar to those of    formula I of the present invention.

The measurement was carried out at different wavelengths of incidentlight and with the viewing angle varying in directions parallel andperpendicular to the major optical axis of the retardation film, whichcorresponds to the direction of orientation of the polymerized mesogenicmaterial or, in case of the stretched PVA and PC films, the stretchdirection.

FIG. 3 a shows the retardation versus viewing angle at a wavelength of550 nm for the optical retardation film of example 1 (curve 4), a PVAfilm (1), a PC film (2) and a film consisting of polymerized mesogenicmaterial with planar alignment (3), wherein the direction of observationis parallel to the major optical axis of the films.

FIG. 3 b shows the retardation versus viewing angle at a wavelength of550 nm for the optical retardation film of example 1 (curve 4), a PVAfilm (1), a PC film (2) and a film consisting of polymerized mesogenicmaterial with planar alignment (3), wherein the direction of observationis perpendicular to the major optical axis of the films.

The inventive optical retardation film exhibits a retardation with asignificantly low viewing angle dependence, in particular when comparedto a stretched PVA film.

FIG. 4 shows the normalized retardation versus wavelength of aninventive optical retardation film (curve 4) compared the state of theart optical retardation films 1, 2 and 3 mentioned above (curve 1 to 3).

From FIG. 4 it can be seen that the wavelength dependence of theretardation of an inventive optical retardation film is very low and issecond only to the PVA film, which, however, shows an unfavourableviewing angle dependence, as depicted in FIGS. 3 a/b.

Thus, when taking the combination of wavelength dependence and viewingangle dependence of the retardation, the inventive optical retardationfilm shows superior behaviour compared to state of the art opticalretardation films. Such a film is therefore suitable for the use in aliquid crystal display device.

Example 2

The following polymerizable mixture was formulated

-   compound(1) 38.4%-   compound (2) 38.4%-   compound (4) 19.2%-   Irgacure 907 4.0%

An optical retardation film is prepared from a solution of 20% by weightof the above given polymerizable mixture in toluene as described inexample 1.

Use Example B

The retardation of the optical retardation film of example 1 wasmeasured as described in example A. The results are depicted in FIG. 5,that shows the retardation versus viewing angle at a wavelength of 550nm, with the direction of observation being parallel (curve a) andperpendicular (curve b) to the major optical axis of the film. For awavelength of 550 nm and normal incidence the retardation is 100 nm.

The optical performance of the inventive retardation film was determinedwhen used together with a broad band cholesteric circular reflectivepolarizer that is consisting of a polymerized mixture comprising chiraland achiral reactive mesogenic compounds. The reflective polarizerexhibits a cholesteric structure with planar orientation and multiplepitch lengths of the cholesteric helix, and has a broad wavelengthreflection band with a bandwidth of 300 nm.

The luminance of light from a commercial LCD backlight passing throughan assembly with the circular reflective polarizer and the inventiveoptical retardation film of example 2 was measured using a MinoltaCS-100 colour camera at a range of viewing angles (−60° to +60°). Theexperiment was repeated with a similar assembly, wherein the inventiveoptical quarter wave retardation film was replaced by one of the stateof the art retardation films 1, 2 and 3 described in example A.

FIGS. 6 a and 6 b depict the measurement results for observationparallel (6 a) and perpendicular (6 b) to the major optical axis of thefilms.

Curve 5 in FIGS. 6 a/b depicts the luminance of the LCD backlight andthe circular reflective polarizer alone. Curves 1 to 4 show theluminance of the LCD backlight and a combination of the circularreflective polarizer together with the inventive optical retardationfilm (curve 4) or one of the state of the art retardation films 1 to 3(curve 1 to 3, each corresponding to the film with the same number).

When viewing in a direction parallel to the major optical axis of thefilms, the luminance of an inventive assembly comprising the inventiveoptical retardation film of example 2 is slightly lower than that of theassembly comprising the state of the art films 1 to 3.

However, when viewing in a direction perpendicular to the major opticalaxis of the films, the luminance of an inventive assembly comprising theinventive optical retardation film of example 2 is slightly lower thanthat of the assembly comprising the state of the art films 1 to 3 forsmall viewing angles, but higher for viewing angles larger than 30degrees. The cross-over angle ac for an inventive assembly is increased.

The optical retardation film and the broadband circular reflectivepolarizer in FIGS. 6 a/b according to example B are not opticallycoupled. If they are laminated together, or if the circular reflectivepolarizer is prepared by polymerization of a mixture of reactivecholesteric mesogenic compounds using the optical retardation film as asubstrate, the cross over angle ac can be further increased.

The results of experiments according to example A and B clearlydemonstrate the improved properties of an inventive optical retardationfilm compared to an optical retardation film of the state of the art,especially when used in combination with a broadband cholestericcircular reflective polarizer.

The preceding examples can be repeated with similar success bysubstituting the generically or specifically described reactants and/oroperating conditions of this invention for those used in the precedingexamples.

From the foregoing description, one skilled in the art can easilyascertain the essential characteristics of this invention, and withoutdeparting from the spirit and scope thereof, can make various changesand modifications of the invention to adapt it to various usages andconditions.

1. Polymer layers comprising an anisotropic polymer layer exhibiting atilted structure with an optical axis having a tilt angle θ relative tothe plane of the layer greater than zero, obtained by polymerizing apolymerizable mesogenic material comprising at least one compound of theformula:P-(Sp-X)_(n)-MG-R  I wherein P is a polymenzable group, Sp is a spacergroup having 1 to 20 C atoms, X is a group of —O—, —S—, —CO—, —COO—,—OCO—, —OCOO— or a single bond, n is 0 or 1, MG is a mesogenic ormesogenicity supporting group, and R is an alkyl radical with up to 25 Catoms optionally unsubstituted, mono- or polysubstituted by halogen orCN, optionally one or more non-adjacent CH₂ groups are replaced,independently, by —O—, —S—, —NH—, —N(CH₃)—, —CO—, —COO—, —OCO—, —OCO—O—,—S—CO—, —CO—S— or —C≡C— where oxygen atoms are not linked directly toone another, or R is halogen, cyano or, independently, P-(Sp-X)_(n)— asdefined in formula I, wherein the polymerizable mesogenic materialcomprises at least 95% by weight of polymerizable compounds, and thetilt angle θ in each of said layers varies continuously in a directionnormal to the layer, starting from a minimum value θ_(min) of 0-20degrees the side of the layer facing the other layer or, if present, thecommon substrate, and ranging to a maximum value θ_(max) of 20-90degrees on the opposite side of the layer.
 2. Polymer layers accordingto claim 1, wherein the polymerizable mesogenic material is a mixtureof: a1A) 10 to 65%, by weight of at least one compound of formula Ihaving one polymerizable group, wherein R is an alkyl or alkoxy groupwith 1 to 12 C atoms; a1B) 5 to 40% by weight of at least one compoundof formula I having one polymerizable group, wherein R is CN, F, Cl or ahalogenated alkyl or alkoxy group with 1 to 12 C atoms; a2) 2 to 90% byweight of at least one compound of formula I having two polymerizablegroups, wherein R has one of the meanings of P-(Sp-X—)_(n); and b) 0.01to 5% by weight of an initiator.
 3. Polymer layers according to claim 2,wherein 10-65%, by weight of at least one compound of formula I is ofthe formula:

wherein x is 1 to 12, R² is C₁₋₁₂ alkyl or alkoxy, and A⁴ is1,4-phenylene, trans-1,4-cyclohexylene or a single bond.
 4. Ananisotropic polymer layer exhibiting a tilted structure with an opticalaxis having a tilt angle θ relative to the plane of the layer greaterthan zero, obtained by polymerizing a polymerizable mesogenic materialcomprising at least one compound of the formula:P-(Sp-X)_(n)-MG-R  I wherein P is a polymerizable group, Sp is a spacergroup having 1 to 20 C atoms. X is a group of —O—, —S—, —CO—, —COO—,—OCO—, —OCOO— or a single bond, n is 0 or 1, MG is a mesogenic ormesogenicity supporting group, and R is an alkyl radical with up to 25 Catoms optionally unsubstituted, mono- or polysubstituted by halogen orCN, optionally one or more non-adjacent CH₂ groups are replaced,independently, by —O—, —S—, —NH—, —N(CH₃)—, —CO—, —COO—, —OCO—, —OCO—O—,—S—CO—, —CO—S— or —C≡C— where oxygen atoms are not linked directly toone another, or R is halogen, cyano or, independently, P-(Sp-X)_(n)— asdefined in formula I; wherein the polymerizable mesogenic materialcomprises at least 95% by weight of polymerizable compounds, and thetilt angle θ in said layer varies continuously in a direction normal tothe layer, starting from a minimum value θ_(min) of 0-20 degrees at oneside of the layer and ranging to a maximum value θ_(max) of 20-90degrees on the opposite side of the layer.
 5. A polymer layer accordingto claim 4, wherein the polymerizable material comprises at least onecompound of formula I having one polymerizable group and at least onecompound of formula I having two polymerizable groups.
 6. A polymerlayer according to claim 4, wherein the polymerizable material comprisesat least one compound of formula I wherein the mesogenic group MG is ofthe formula:

where L is: F, Cl, CN, or a fluorinated alkyl, alkoxy or alkanoyl groupwith 1 to 4 C atoms, and r is 0, or
 2. 7. A polymer layer according toclaim 4, wherein the polymerizable material comprises at least onecompound of formula I where P is: CH₂═CW—COO—, WCH═CH—O—,

 or CH₂═CH-Phenyl-(O)_(k)— with W being H, CH₃ or Cl and k being 0 or 1.8. A polymer layer according to claim 4, wherein the polymerizablemesogenic material comprises at least one compound of formula:

wherein x and y are, independently, 1 to 12, A is a 1,4-phenylene or1,4-cyclohexylene group, R¹ is halogen, cyano or an optionallyhalogenated alkyl or alkoxy group with 1 to 12 C atoms, and L¹ and L²are, independently. H, F, Cl, CN, or a halogenated alkyl, alkoxy, oralkanoyl group with 1 to 7 C atoms.
 9. A polymer layer according toclaim 4, wherein the polymerizable material comprises at least onecompound of the formula:

wherein x is 1 to 12, R² is C₁₋₁₂ alkyl or alkoxy, and A⁴ is1,4-phenylene, trans-1,4-cyclohexylene or a single bond; at least onedireactive compound of formula I; and at least one dielectricallypositive monoreactive compound of formula I.
 10. A polymer layeraccording to claim 4, wherein the polymerizable mesogenic material is amixture of: a1) 10 to 99% by weight of at least one mesogen according toformula I having one polymerizable functional group, a2) 0 to 70% byweight of at least one mesogen according to formula I having two or morepolymerizable functional groups, and b) 0.01 to 5% by weight of aninitiator.
 11. A liquid crystal display comprising a display cell and atleast one polymer layer according to claim
 4. 12. A polymer layeraccording to claim 4, wherein the mesogenic or mesogenicity supportinggroup is a compound of formula:-(A¹-Z¹)_(m)-A²-Z²-A³-  II wherein A¹, A² and A³ are, independently,1,4-phenylene where one or more CH groups optionally replaced by N,1,4-cyclohexylene, optionally, one or two non-adjacent CH₂ groups arereplaced by O and/or S, 1,4-cyclohexenylene or naphthalene-2,6-diyl,optionally these groups are unsubstituted, mono- or polysubstituted witha halogen, a cyano, or a nitro group, or an alkyl, alkoxy or alkanoylgroup having 1 to 7 C atoms, wherein one or H atoms may be substitutedby F or Cl, Z¹ and Z² are each, independently, —COO—, —OCO—, —CH₂CH₂—,—OCH₂—, —CH₂O—, —CH═CH—, —C≡C—, —CH═CH—COO—, —OCO—CH═CH— or a singlebond and m is 0, 1 or
 2. 13. A polymer layer according to claim 4,wherein n=1.
 14. A polymer layer according to claim 4, wherein the tiltangle θ is 5-80° and the polymerizable mesogenic material comprises atleast 96% by weight of polymerizable compounds.
 15. A polymer layeraccording to claim 4, wherein the at least 95% by weight ofpolymerizable compounds are of the formula I.
 16. A polymer layeraccording to claim 4, wherein the polymer layer is untwisted.
 17. Apolymer layer according to claim 4, wherein the polymerizable materialcomprises 1 to 80% by weight of at least one dielectrically positivemonoreactive mesogenic compound.
 18. A polymer layer according to claim17, wherein said dielectrically positive monoreactive mesogenic compoundhas a dielectric anisotropy Δ∈>1.5.
 19. A polymer layer according toclaim 17, wherein said dielectrically positive monoreactive mesogeniccompound has a polar terminal group of CN, F, Cl, OCF₃, OCF₂H, OC₂F₅,CF₃, OCN or SCN.
 20. An anisotropic polymer layer exhibiting a tiltedstructure with an optical axis having a tilt angle θ relative to theplane of the layer greater than zero, obtained by polymerizing apolymerizable mesogenic material comprising at least one compound of theformula:P-(Sp-X)_(n)-MG-R  I wherein P is a polymerizable group, Sp is a spacergroup having 1 to 20 C atoms, X is a group of —O—, —S—, —CO—, —COO—,—OCO—, —OCOO— or a single bond, n is 0 or 1, MG is a mesogenic ormesogenicity supporting group, and R is an alkyl radical with up to 25 Catoms optionally unsubstituted, mono- or polysubstituted by halogen orCN, optionally one or more non-adjacent CH₂ groups are replaced,independently, by —O—, —S—, —NH—, —N(CH₃)—, —CO—, —COO—, —OCO—, —OCO—O—,—S—CO—, —CO—S— or —C═C— where oxygen atoms are not linked directly toone another, or R is halogen, cyano or, independently, P-(Sp-X)_(n)— asdefined in formula I; wherein the polymerizable mesogenic materialcomprises at least 95% by weight of polymerizable compounds and whereinthe polymerizable mesogenic material is a mixture of: a1A) 10 to 65%, byweight of at least one compound of formula I is of the formula:

 wherein x is 1 to 12, R² is C₁₋₁₂ alkyl or alkoxy, and A⁴ is1,4-phenylene, trans-1,4-cyclohexylene or a single bond; a1B) 5 to 40%by weight of at least one compound of formula I having one polymerizablegroup, wherein R is CN, F, Cl or a halogenated alkyl or alkoxy groupwith 1 to 12 C atoms; a2) 2 to 90% by weight of at least one compound offormula I having two polymerizable groups, wherein R has one of themeanings of P-(Sp-X—)_(n); and b) 0.01 to 5% by weight of an initiator.